1. The effects of methanethiol (MT), DL-dithiothreitol (DTT) and N-ethyl-maleimide (NEM) were studied on the electrical activities of the frog cord. 2. Methanethiol depressed spontaneous dorsal and ventral root potentials, whereas no effects were observed on evoked responses. 3. N-ethyl-maleimide, a SH modifying agent, irreversibly depressed electrical cord activities. 4. DL-Dithiothreitol, a SH reducing agent, dramatically increased spontaneous electrical cord pattern. 5. It is suggested that interneuronal membrane sulphydryl groups of dorsal horn cell population are involved in the origin of spontaneous electrical cord activities and that MT interacts with these interneurones, probably oxidizing membrane SH groups.